Magnetron starting circuit

ABSTRACT

A circuit for and method of operating microwave oven apparatus is disclosed including means for the momentary interruption of the starting cycle at a predetermined time interval. Such operation will shift the operating mode of the energy generator as well as the voltage level from a higher to a lower frequency to prolong life and enhance efficiency. The invention is particularly suited for permanent magnet type energy generators for use in microwave oven apparatus where continuous operation is utilized. The invention may be implemented in all types of microwave oven apparatus simply and economically.

United States Patent [1 1 Peterson Aug. 14, 1973 MAGNETRON STARTING CIRCUIT 1 Primary Examiner-R. F. Staubly 7 I t Donal E. te l t l 5] men or d Fe owa Cl y Iowa Assistant Examiner-Hugh D. Jaeger Assigneer mm g ra n, ma Attorney-Harold A. Mur h Edgar 0. Rest et al.

Iowa

[22] Filed: Oct. 1, 1971 [57] ABSTRACT [211 Appl. No.: 185,624 A circuit for and method of operating microwave oven apparatus is disclosed including means for the momentary interruption of the starting cycle at a predeter- [52] US. Cl. 219/10.55, 331/91 mined time intervaL such operation will Shift the open irnt. Cl. i g d f the gy generator as we" as the volb I 1 of Search g level f a gh to a lower frequency to p g 331/91 315/3951 39'55 life and enhance efficiency. The invention is particularly suited for pennanent magnet type energy genera- [5 6] Reference Cited tors for use in microwave oven apparatus where contin- UN TED STA ES PATENTS uous operation is utilized. The invention may be imple- 3,480,753 11/1969 Wilson et a] 2l9/l0.55 mented in all types of microwave oven apparatus sim- 3,61l,027 10/1971 Koinuma.... 331/91 X ply and economically. 3,569,656 3/1971 White 219/1055 3,470,942 10/1969 Fukada et al. 219/1055 x 1 Claim, 6 Drawing Flsures 3 Sheets-Sheet 1 Patented Aug. 14, I973 3,752 91gg;

3 Sheets-Sheet 2 Patented Aug. 14, 1973 3 Sheets-Sheet 5 l I l I MAGNETRON ENERGY GENERATOR ASSEMBLY HIGH VOLTAGE SUPPLY MAGNETRON STARTING CIRCUIT BACKGROUND OF THE INVENTION The invention relates to electrical circuits for and a method of operating microwave oven apparatus having a magnetron energy generator.

The generator which has received wide usage in the generation of high frequency electromagnetic oscillations for cooking is the magnetron. Such devices are energized by main line AC voltages and power supplies capable of generating rectified DC voltages at levels of, for example, 4 to 6 kilovolts. Crossed electric and magnetic fields extend within an interaction region defined between a central cathode assembly and circumferentially disposed anode cavity resonators. Electrons emitted by the heated cathode are accelerated towards the cavity resonators and rotate in a substantially helical path to form a rotating spoke-like space charge. The electrons interact in energy-exchanging relationship with the electric fields in the cavity resonators to generate extremely high power electrical energy oscillations. The intense magnetic fields required for the operation of the magnetron oscillator are conventionally provided by external permanent magnets or an electromagnet. The starting voltage applied between the anode and cathode structures is generated by means of step-up and DC rectification of the AC line voltage. The device is inherently a positive reactance structure that oscillations are generated only when the anode cavity resonator structure is positive relative to-the cathode on alternate half cycles. Rectifying circuits are, therefore, employed in combination with a high voltage transformer to render the magnetron operative. The unidirectional characteristic creates numerous circuit and power supply problems for regulation of current and voltage, particularly for the microwave oven applications where the magnetron is continuously operated. Further details relative to devices of the type under consideration may be had from the text Microwave Magnetrons, Radiation Laboratories Series, Vol. 6, by G. B. Collins, McGraw-Hill Book Company, Inc., 1948.

At the beginning of each cycle of operation the high voltage is applied between the anode and cathode by such means as a power supply circuit of the type disclosed in US. letters Pat. No. 3,396,342, issued Aug. 6, 1968 to A. E. Feinberg. Such a circuit includes a high leakage reactance constant current transformer having a primary winding connected to the AC line voltages of 120 or 220 volts andfrequencies of 60 hertz. A secondary winding is inductively coupled to the primary winding with full wave rectifying circuit means connected in series with this secondarywinding and the cathode assembly. A separate transformer for preheating the cathode filament has also been provided in numerous prior art devices to preheat this element before the full voltage is applied. From the operator's standpoint, however, it is desirable to operate microwave oven apparatus with as fast a starting period as possible. After the magnetron generator has been fully energized the oscillations will commence at a particular point along the voltage-current curve and become selfsustaining. The oscillations, however, are desired in only one mode at a time and for the continuously operated application it is important that the device operate initially in the correct mode with the accompanying voltages and current values and that operation be continued in the desired mode.

A problem arises in such circuits with the commencement of the application of the high voltage across the anode and cathode to the open circuit voltage value. The cathode filament begins to conduct current thereby loading the circuit and the voltage drops slightly to the point at which the oscillations commence. Continued operation at this point, however, which is normally characterized by high frequency and high voltage levels, will result in unstable operation. The very high temperatures will shorten cathode life and increase the necessity for replacement of the magnetron generator. Since the magnetrons are designed to operate at lower voltages for longer life, it is desirable that lower order oscillating mode be utilized. A mode shift, therefore, is desirably instituted after the high order mode is stimulated by reason of the initial high open circuit transient voltage. Such transient voltage can run as high as 12 to 15 kilovolts which is substantially above the normal operating range of 4 to 6 kilovolts. As an example of the problem encountered in the continuous operation of magnetron oscillators a high order mode of, illustratively, 5/3/8 could be initially excited as a result of the very high transient voltage when the desired mode of oscillation is the 4/4/8 or pi" mode as it is commonly referred to in the art.

SUMMARY OF THE INVENTION In accordance with the teachings of the invention, an electrical circuit for and method of operating a microwave oven apparatus is disclosed having a momentary interruption at a predetermined time interval after the commencement of the starting cycle. The high voltage across the anode and cathode of the generator with a full wave or half wave voltage doubler circuit rises to its customary level to initiate current conduction at this point and the device has a tendency to operate in the higher and less efficient mode. A momentary interruption of the circuit is now provided by suitable means such as, for example, a delay timer to result in the voltage across the energy generator falling towards zero. At a predetermined time in the order of 0.1 to 0.4 seconds the source voltage is re-applied. The filament of the cathode which was initially heated by the starting voltage remains sufficiently hot to immediately commence conduction. The oscillations now begin on the rising side of the pulse at a more stable and efiicient low voltage level representative of the new operating mode.

The energy generator continues operating in this mode until the end of the operating cycle when the magnetron becomes inoperative.

BRIEF DESCRIPTION OF THE DRAWINGS Details of the invention will be readily understood after consideration of the following description and reference to the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of the illustrative circuit of the invention;

FIG. 2 is a diagram of the El characteristics of a magnetron energy generator of the permanent magnet type;

FIG. 3 is a representation of a voltage waveform plotted on a time scale illustrative of the operation of a magnetron energy generator;

FIG. 4 is an isometric view of a microwave oven apparatus embodying the invention;

FIG. 5 is a vertical cross-sectional view of the microwave oven apparatus illustrated in FIG. 4; and

FIG. 6 is a side elevational view of a magnetron energy generator utilized in the illustrative microwave oven apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT Before proceeding to a description of the specific details of the emboidment of the invention, an illustrative microwave oven apparatus 20 will be described with reference being directed to FIGS. 4 and 5. Magnetron type generator 12 which will be described in greater detail hereinafterradiates at microwave frequencies of- 2,450 megahertz corresponding to a wavelength of approximately five inches in space. For the purpose of the present description the term microwaves refers to electromagnetic radiation in that portion of the spectrum having wavelengths of from 1 meter to l millimeter and frequencies in excess of 300 MHz. Rectangular conductive walls 14 define a heating enclosure 16 having an access opening closed by means of a door assembly l8 actuated by handle 20. v

Control panel 22 is disposed adjacent to the access opening with the electrogmagnetic energy generator means and high voltage power supply circuit disposed behind. Panel 22 supports a 5-minute timer switch 24 as well as a longer or 30-minute timer switch 26. A start control button 28, stop control button 30 and light control button 32 are also mounted on the panel 22.

The magnetron generator 12 is coupled to the high voltage supply and electrical controls indicated generally by block 34. The microwave energy is fed from the generator by means of an antenna 36 disposed within a dielectric dome member 38 into a launching rectangular waveguide section 40 adapted to propagate the energy the desired frequency for distribution within the enclosure 16. Waveguide 40 is short-circuited at one end by a wall member 42 and is open at the inner end 44. This energy may be efficiently distributed by any of the means well known in the art such as, for example,

. a stirrer 46 having a plurality of paddle members 48 actuated by a motor 50. The items to be heated or processed are supported on a dielectric plate 52 within the enclosure 16.

A typical magnetron energy generator 12 is shown in FIG. 6. A conductive envelope 54 comprising the anode has a plurality of circumferentially disposed cavity resonators as well as a filament controlled central cathode assembly. Magnetic field producing means 56 in this embodiment are of the permanent magnet type with the magnetic field shaped by members 58 and 60. A substantially U-shaped support and housing member 62 surrounds the magnets and forms the magnetic field return path. The antenna member 36 and dome member 38 protrude through the housing member 62. High voltage electrical leads 64 and 66 are coupled to the high voltage circuit through an RF shielded bypass capacitor means housed within a box-like member 68. This component is secured to a plate member 70 which is in turn secured to the housing member 62. Radiating fins 72 contact the conductive envelope 54 to provide for rapid cooling as by circulating air to remove the heat generated within the device by the high frequency oscillations.

Referring now to FIG. 1 the high voltage electrical circuit for operation of the microwave oven apparatus as well as the new method of operation will be described. An AC source 74 of conventional main line voltages is coupled by means of connecting lines 76 and 78 to the high voltage supply circuit through a manually operated stop-start switch 80. A leakage reactance transformer 82 has primary winding 84 connected across the main line leads. The secondary winding 86 is inductively coupled to the primary winding and cutomarily in the step-up tranformers utilized in such apparatus a turns ratio of 40-50:l is utilized. A fullwave voltage doubler circuit 88 is serially connected to the secondary winding 86 to provide the high electric fields required for the cathode 90 of the magnetron device 12 with the envelope 54 grounded as at 92. The cathode is commonly of the direct heated type, such as thoriated tungsten which has been carbonized. To achieve the electron emission temperatures approximately 3.0 volts AC is directly applied to the filament by means of a few turns of the secondary winding 86a.

The magnetron energy generator 12 hereinbefore described in connection with FIG. 6 also defines a plurality of cavity resonators 94 circumferentially disposed about the cathode 90. The high frequency oscillations are coupled from the generator 12 by means of a coupling loop 96 to the antenna.

Full-wave voltage doulber circuit 88 includes diodes 98 and 100 together with the capacitors 102 and 104.

In operation, when the end of the secondary winding 86 connected to lead 106 is. poled positive, the current flows through semiconductor diode 100 to charge capacitor .104 to the peakinput voltage of a predetermined value. Semiconductor diode 98 remains nonconductive. On the next half-cycle, lead 108 becomes positive and the current cannot flow through diode 100. Capacitor 102 becomes charged to the peak input voltage with the diode 98 being conductive. The total output rectified voltage across the anode and cathode of the magnetron generator 12 is the sum of the charges or approximately twice the voltage across each capacitor 102 and 104. In the practice of the invention, other DC rectifying circuits may be employed such as halfwave voltage doublers as well as voltage triplers or quadruplers. It will be appreciated that since the magnetron generator 12 is essentially a unidirectional device, it provides rectification for part of each cycle when the anode is positive relative to the cathode.

In view of the certain extremely high open circuit surge voltages which are encountered upon the closing of the main switch which can be as high as 12 to 15 kilovolts, the application of the full line voltage to the primary lead 84 is preferred in a step manner with an appropriate serially connected resistor 110 introduced initially. Electromechanical relay means 112 provide for the simultaneous making and breaking of this portion of the primary circuit across dual contacts 114 and 116 by means of blade member 118. When the contacts are open a resistor 110 having, illustratively a value of 10 ohms introduces additional resistance during the first stage of starting period. Numerous relay actuation means 112 may be incorporated including an R-C circuit with a semiconductor diode resistor and capacitor or other suitable means to step the application of the full line voltages during the initial turn-on period of the circuit.

Referring now to FIG. 2 an explanation of the prior art phenomenon with relation to the operating mode will be described. Upon closing of the main switch 80 the voltage across the anode and cathode reaches a point A which may typically be in the range of 7 to 8 kilovolts. The plot of the E-[ characteristics for the magnetron device is indicated by line 120. As the cathode filament becomes heated the generator begins to conduct current with the voltage dropping to point B. Line 122 indicates the high order mode oscillatingstate which can have a frequency of approximately 4,420 MHz which is substantially higher than the 2,450 MHz required for the normal operation of microwave oven apparatus. Continued operation of the generator in the higher frequency mode with the accompanying much higher voltages results in extremely short life for the cathode unstable operation. A shift therefore to a more desirable operating mode, for example, that indicated by the line 124 and point C is required. In an exemplary embodiment it was discovered that with a momentary interruption in the starting cycle provided, for example, by an automatic delay timer 126 operating a switch 128, the voltages across the magnetron generator will not drop immediately to a zero level and the filament remains sufficiently hot to reinstitute conduction upon the reapplication of the full line voltages. The momentary interruption in the starting cycle has resulted in themode shift to a new operating level when the interruption takes place at a period in the order of 3-5 seconds after the circuit is energized. The duration of the momentary interruption is characteristically in the order of 0.1 to 0.4 seconds-before reapplication of the full line voltages.

Referring now to FIG. 3 the results of the novel electrical circuit and method of operating the magnetron microwave oven apparatus are demonstrated. In this illustration the oscillations are plotted at the respective voltage levels with time as the base reference. Initially, with the voltage levels at the point A a point is reached indicated by the letter B at which the pulse waveform 132 of the steady oscillation state results. Continued operation at this high voltage level only results in the overheating of the cathode and shortened life of the energy generator. The momentary interruption indicated by spike 130 results in pulse waveform 134 at the new lower voltage level indicated at point C. The steady oscillations at this voltage level reflect the desired mode of operation with accompanying stability and longer life inherent in the practice of the invention. Operation remains at this level until the high voltage circuit is once again de-energized by the opening of main switch as, for example, at the end of the work period or power failure.

Numerous other means for the introduction of the momentary interruption in the primary circuit will be evident to those skilled in the art. It is intended, therefore, that the foregoing illustrative embodiment and detailed description be considered in its broadest aspects and not in a limiting sense.

I claim:

1. Microwave oven apparatus comprising:

an enclosure defined by conductive walls;

a microwave energy generator adapted toradiate energy within said enclosure;

electrical circuit means for energizing said generator including line voltage source means and high voltage transformer means having primary and secondary windings with said primary winding connected to said source means;

mean for energizing said circuit including a first manually operated siwtch; and

means coupled to said primary winding including a secind switch and automatic delay timer means connected to said second switch for opening and closing said energized circuit for a predetermined time interval of 0.1 to 0.4 seconds approximately 3-5 seconds after said first switch is closed. .v i. .'i i 

1. Microwave oven apparatus comprising: an enclosure defined by conductive walls; a microwave energy generator adapted to radiate energy within said enclosure; electrical circuit means for energizing said generator including line voltage source means and high voltage transformer means having primary and secondary windings with said primary winding connected to said source means; mean for energizing said circuit including a first manually operated siwtch; and means coupled to said primary winding including a secind switch and automatic delay timer means connected to said second switch for opening and closing said energized circuit for a predetermined time interval of 0.1 to 0.4 seconds approximately 3-5 seconds after said first switch is closed. 